In the copending U.S. Application of J. C. Rosenfeld Ser. No. 045,464 filed of even date with this application there are described linear aromatic polyesters comprising bisphenol and dicarboxylic acid monomer units which terminate in carboxylic acid ester groups derived from a long chain (C.sub.8 -C.sub.45) aliphatic monohydric alcohol, e.g. stearyl alcohol. These polyesters are characterized by enhanced hydrolytic stability compared to corresponding polyesters having conventional substituents terminating the polyester chain (such as --OH, COOH, --COOR where R is, for example, an aryl group, e.g. phenyl).
According to the aforementioned copending application, the linear aromatic polyesters can be prepared by carrying out transesterification polymerization of a mixture of a diaryl ester of a dicarboxylic acid, a bisphenol and a minor molar proportion (based on the diaryl ester reactant) of a C.sub.8 -C.sub.45 aliphatic monohydroxy alcohol to provide the aforementioned terminal or end cap substituents in the polyester product. In the aforementioned reaction a monohydroxy aryl compound is displaced from the diaryl ester by the bisphenol and is distilled from the polymerization mass. According to a preferred embodiment of the aforementioned process duration of heating of the polymerization reaction mass is curtailed to avoid a decomposition side reaction of the long chain alkyl-carboxylate terminated polyester (resulting in loss of the long chain alkyl substituent, e.g. a stearyl group, via formation of olefin e.g. 1-octadecene) which side reaction results in loss of the desired long chain alkyl ester termination in the polyester product.
It is now found that in the aforementioned preparatory process, involving the simultaneous transesterification reaction of the diaryl dicarboxylic acid ester, the bisphenol and the long chain aliphatic alcohol a substantial loss of the long chain alcohol from the preparatory reaction mass occurs due to entrainment of the alcohol with the aromatic monohydroxy compound which distills from the reaction mass, i.e. a substantial quantity of the alcohol reactant is lost as distillate without reacting to form the desired alkyl carboxylate end cap substituents of the polyester.
The loss of long chain alcohol from the reaction mass results in a polyester product having a relatively low percentage of the desired long chain alkyl end cap substituents. While this loss can be overcome by providing efficient fractionation of the distillation overhead stream containing the monohydroxy aromatic compound and the long chain alcohol, efficient fractional distillation requires costly equipment and is necessarily very time consuming. Charging of an excess of the alcohol to compensate for the loss of alcohol is not desirable since the alcohol is costly and use of such excess may deleteriously lower the molecular weight of the product. Separation and recycling of the alcohol from the distillate to the reaction mass to overcome the aforementioned loss is both inconvenient and costly.
It is the object of the present invention to devise an efficient improved process of preparing the present polyesters which substantially diminishes the loss of the long chain alcoholic reactant via entrainment with the monohydroxy aromatic compound side product evolved in the transesterification polymerization.